Multi-effect and single effect distillation processes have long been used to produce distillate from feed water streams containing dissolved materials. These processes have been used for the production of desalinated (distillate) water from seawater (salt water) and the like. Basically, such distillation processes use one or a series of vessels (effects) and use the principles of evaporation and condensation at reduced ambient pressure in the various effects. This permits a feed water stream to undergo evaporation without the need to supply additional heat after the first effect.
Generally an effect consists of a vessel, a heat exchanger and devices for transporting various fluids between the effects. Processes using a single effect are also known. Diverse designs have been used for the heat exchanger, such as horizontal tubes with a falling water film on the outside, vertical tubes with a falling water film on the inside, or plates with a falling water film and the like.
There are various methods for adding feed water to a multi-effect distillation system. In one embodiment, the feed water is added in equal portions to the various effects. The feed water is sprayed or otherwise distributed onto the evaporator surfaces in an effect in a thin film to promote rapid evaporation after it has been preheated. The surfaces in the first effect may be heated by steam or any other suitable heating fluid or method, such as vapor compression. The distillate vapor produced in the last effect may be condensed. This condenser may be cooled by an inlet water containing feed and cooling water. In this way the feed water is heated by means considered internal to the desalination process.
Only a portion of the feed water applied to the heat transfer surfaces is evaporated. The remaining feed water in each effect is concentrated and may be discarded or fed to the next effect where some of the brine flashes into distillate vapor. This vapor is also part of the heating process. All water evaporated and subsequently condensed inside the effects is a distillate product.
The reduced pressure in each of the effects in the multi-effect distillation process is maintained as well known to the art. The thermal efficiency of the process depends in part upon the number of effects, with from 1 to about 16 effects being found in some plants. Desirably, temperatures in the first effect are from about 70 to about 160° F., which reduces the potential for scaling of some feed water streams on the heat transfer surfaces. High temperature additives or other processes, such as nano-filtration, may permit higher temperatures.
Highly efficient multi-effect distillation plants require a considerable number of effects and large heat transfer areas and are therefore used in situations where energy costs are high. In cases where low cost steam is available, the number of distillation effects and corresponding capital costs are significantly reduced. In some multi-effect distillation applications, a thermal vapor compression cycle may be added to the system. If desired, mechanical vapor compression systems may be substituted. This can considerably reduce the number of effects and the surface area required for the same capacity.
In many instances the cost of steam used for heating is a significant portion of the overall expense of the total process operating costs. Accordingly, a continuing effort has been directed to the development of methods which allow optimization of the use of high quality steam used for heating the heat exchange surfaces of the first effect in the distillation process. While some heating can be achieved by heat exchanging an inlet water stream containing feed and cooling water with the produced distillate vapor in a condenser, the temperature of the feed water leaving the condenser is below the level desired for efficient operation of the process. Accordingly, this feed water may be preheated with distillate vapor produced in the distillation process to bring it to a suitable temperature for use in the effects. As indicated previously, it would be highly desirable to optimize the efficiency of the use of high quality steam by recovering as much distillate as possible per unit of high quality steam. Further, the feed water stream may be heated by other internal means, such as heat exchange with the condensate, brine, inter and after condensers from a vacuum system, pumps, or the like, or the feed water may be fed without heating to the effects.